Zinc-aluminum hot dip coated ferrous article

Abstract

An improved hot dip coating of the zinc aluminum type contains germanium (Ge). In Zn-10Al coatings the addition of 0.1-1.2 Ge weight percent increases the fluidity of the molten metal in the bath and thereby enables substantially thinner coatings having more refined phase structure. Silicon (Si) is found to exhibit an analogous effect, but it not as powerful as Ge. Improved coatings contain zinc, 7-17Al, and 0.1-3(Ge+Si), where Ge ranges from 0-1.2, all weight percent.

We claim: 
     
       1. A ferrous metal article having on the surface thereof a hot dip coating comprised predominantly of zinc (Zn), up to 17 weight percent aluminum (Al), and at least 0.1 weight percent germanium (Ge). 
     
     
       2. The article of claim 1 wherein the coating is more particularly characterized by weight percent as 4-17Al, 0.1-1.2Ge, balance Zn. 
     
     
       3. The article of claim 2 wherein the coating is more particularly characterized as 7-13Al, 0.1-1Ge balance Zn. 
     
     
       4. The article of claim 3 wherein the coating consists essentially of 9-11Al, 0.1-0.3Ge, balance Zn. 
     
     
       5. The article of claims 1, 2, 3 or 4, said coating containing 0.1-3 weight percent silicon (Si) in substitution for Zn. 
     
     
       6. The article of claims 1, 2 or 3 having a nominal alpha (aluminum rich) phase of a size of less than about 15×10 -6  m in dimension when applied to a small ferrous workpiece using inertial removal of excess hot dip coating. 
     
     
       7. The article of claim 6 wherein the alpha phase size is predominantely of the order of 1-2×10 -6  m. 
     
     
       8. In the method of applying zinc aluminum alloy coatings to articles by hot dipping, the improvement which comprises adding 0.1-1.2 weight percent germanium to the molten coating bath. 
     
     
       9. The method of applying an improved zinc aluminum alloy coating on a ferrous metal article, characterized by passing the article through a molten zinc (Zn) base metal bath comprised by weight percent of 4-17 aluminum (Al); and 0.1-3 germanium+silicon (Ge+Si), wherein Ge ranges from 0.1-1.2. 
     
     
       10. The method of claim 9 wherein the molten bath is comprised of 7-13Al, 0.1-1(Ge+Si), balance Zn. 
     
     
       11. The methods of claims 8, 9, or 10 further characterized by covering of the molten coating bath into which the article is immersed with a molten halide flux, and holding the bath in the temperature range of 400° to 650° C.